Ferroelectric film and method for manufacturing the same

ABSTRACT

To produce a ferroelectric film formed of a lead-free material. The ferroelectric film according to an aspect of the present invention includes a (K 1−X Na X )NbO 3  film or a BiFeO 3  film having a perovskite structure and a crystalline oxide preferentially oriented to (001) formed on at least one of the upper side and lower side of the (K 1−X Na X )NbO 3  film or BiFeO 3  film, and X satisfies the formula below
 
0.3≦X≦0.7.

TECHNICAL FIELD

The present invention relates to a ferroelectric film and a method formanufacturing the same.

BACKGROUND ART

A conventional method for manufacturing a Pb (Zr, Ti) O₃ (hereinafter,referred to as “PZT”) film will be explained.

On a 4-inch wafer, a Pt film (111)-oriented, for example, is formed, andon the Pt film, a PZT sol-gel solution is spin-coated by a spin coater.Next, the coated PZT sol-gel solution is heated and held on a hot plateto be dried, and the moisture is removed. After that, it is furthermoreheated and held on a hot plate kept at higher temperature to becalcined. The repetition of the process a plurality of times generatesamorphous PZT.

Next, the amorphous PZT having been calcined is subjected to anannealing treatment by using a pressurizing-type lamp annealing device(RTA: rapidly thermal anneal) and PZT crystallization is performed. Thecrystallized PZT film has a perovskite structure. (For example, seePatent Document 1)

On the other hand, PZT has the Tc existing at 300° C. or higher, and hasgood ferroelectric properties and piezoelectric properties, but incircumstances where the whole industrial world aims at making lead free,the problem to be solved is to attain lead-free PZT.

CITATION LIST Patent Document

[Patent Document 1] WO 2006/087777

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As described above, in the industrial world, the production of aferroelectric film formed of a lead-free material is required. An aspectof the present invention aims at producing a ferroelectric film formedof a lead-free material.

Means for Solving the Problems

The following (1) to (17) explain a plurality of aspects of the presentinvention.

(1) A ferroelectric film including:

a (K_(1−x)Na_(x))NbO₃ film or a BiFeO₃ film having a perovskitestructure; and

a crystalline oxide preferentially oriented to (001), formed on at leastone of an upper side and lower side of the (K_(1−x)Na_(x))NbO₃ film orBiFeO₃ film, wherein X satisfies the formula below0.3≦X≦0.7.

(2) The ferroelectric film according to the above (1), wherein thecrystalline oxide is a bismuth layered-structure ferroelectric substancehaving a pseudo-perovskite structure or a tungsten-bronze typeferroelectric substance.

(3) The ferroelectric film according to the above (1) or (2), whereinthe bismuth layered-structure ferroelectric substance is(Bi₂O₂)²⁺(A_(m−1)B_(m)O_(3m+1))²⁻(m=1 to 5) or Bi₂A_(m−1)B_(m)O_(3m+3)(m=1 to 5).

(4) The ferroelectric film according to the above (2) or (3), whereinthe bismuth layered-structure ferroelectric substance is Bi₄Ti₃O₁₂ or(Bi_(4−x)La_(x))Ti₃O₁₂, and x satisfies the formula below0<x<1.

(5) The ferroelectric film according to any one of the above (1) to (4),wherein the crystalline oxide is formed in an island shape or in a filmshape.

(6) The ferroelectric film according to any one of the above (1) to (5),wherein a thickness of the crystalline oxide is 2 to 30 nm.

The thickness of the crystalline oxide referred to here means, when thecrystalline oxide is formed only on one of the upper side and lower sideof the (K_(1−x)Na_(x))NbO₃ film or the BiFeO₃ film, the thicknessthereof, and means, when the crystalline oxide is formed on both theupper side and lower side of the (K_(1−x)Na_(x))NbO₃ film or the BiFeO₃film, the sum of both thicknesses.

(7) The ferroelectric film according to any one of the above (1) to (6),wherein the (K_(1−x)Na_(x))NbO₃ film or BiFeO₃ film is formed by asol-gel method.

(8) The ferroelectric film according to any one of the above (1) to (7),wherein a ferroelectric film including the (K_(1−x)Na_(x))NbO₃ film orBiFeO₃ film, and the crystalline oxide formed on at least one of theupper side and lower side thereof is stacked.

(9) A method for manufacturing a ferroelectric film, including the stepsof:

forming, by coating a sol-gel solution containing K, Na and Nb on asubstrate by a spin-coating method, a coated film on the substrate;

forming, by calcining the coated film, a ferroelectric material film onthe substrate;

forming a first material film for forming a crystalline oxide in anisland shape or in a film shape on the ferroelectric material film; and

forming, by heat-treating the ferroelectric material film and the firstmaterial film for forming a crystalline oxide in an oxygen atmosphere, aferroelectric film obtained by crystallizing the ferroelectric materialfilm and the first material film for forming a crystalline oxide,

wherein a first crystalline oxide obtained by crystallizing the firstmaterial film for forming a crystalline oxide is preferentially orientedto (001).

(10) The method for manufacturing a ferroelectric film according to theabove (9), wherein the first crystalline oxide is a bismuthlayered-structure ferroelectric substance having a pseudo-perovskitestructure or a tungsten-bronze type ferroelectric substance.

(11) The method for manufacturing a ferroelectric film according to theabove (10), wherein the bismuth layered-structure ferroelectricsubstance is Bi₄Ti₃O₁₂ or (Bi_(4−x)La_(x))Ti₃O₁₂, and x satisfies theformula below0<x<1.

(12) The method for manufacturing a ferroelectric film according to anyone of the above (9) to (11), wherein:

before forming a coated film on the substrate, a second material filmfor forming a crystalline oxide preferentially oriented to (001) in anisland shape or in a film shape is formed on the substrate;

the coated film is formed on the second material film for forming acrystalline oxide;

the ferroelectric material film, the first material film for forming acrystalline oxide and the second material film for forming a crystallineoxide are heat-treated in an oxygen atmosphere; and

a second crystalline oxide obtained by crystallizing the second materialfilm for forming a crystalline oxide is preferentially oriented to(001).

(13) A method for forming a ferroelectric film, including the steps of:

forming a first material film for forming a crystalline oxide in anisland shape or in a film shape on a substrate;

forming, by coating a sol-gel solution containing K, Na and Nb on thefirst material film for forming a crystalline oxide by a spin-coatingmethod, a coated film on the first material film for forming acrystalline oxide;

forming, by calcining the coated film, a ferroelectric material film onthe first material film for forming a crystalline oxide;

forming a blocking film on the ferroelectric material film; and

forming, by suppressing separation of K and Na from the ferroelectricmaterial film by the blocking film while heat-treating the ferroelectricmaterial film and the first material film for forming a crystallineoxide in an oxygen atmosphere, a ferroelectric film obtained bycrystallizing the ferroelectric material film and the first materialfilm for forming a crystalline oxide,

wherein a first crystalline oxide obtained by crystallizing the firstmaterial film for forming a crystalline oxide is preferentially orientedto (001).

(14) The method for manufacturing a ferroelectric film according to anyone of the above (9) to (13), wherein the total concentration of the K,Na and Nb contained in the sol-gel solution is from 10 to 50 mol %.

(15) The method for manufacturing a ferroelectric film according to anyone of the above (9) to (14), wherein, by repeating twice or more theformation of the coated film and the calcination when forming theferroelectric material film, a ferroelectric material film including aplurality of coated films is formed.

(16) The method for manufacturing a ferroelectric film according to anyone of the above (9) to (15), wherein the heat treatment is carried outin a pressure range of 0.0993 to 1.986 MPa.

(17) The method for manufacturing a ferroelectric film according to anyone of the above (9) to (16), wherein the ferroelectric film is a(K_(1−x)Na_(x))NbO₃ film having a perovskite structure, and X satisfiesthe formula below0.3≦X≦0.7.

Effect of the Invention

According to an aspect of the present invention, a ferroelectric filmformed of a lead-free material can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing schematically the ferroelectricfilm according to an aspect of the present invention.

FIG. 2 is a drawing showing a crystal structure of BIT (BLT) that is abismuth layered-structure ferroelectric substance.

FIG. 3 is a drawing showing schematically a tungsten-bronze type crystalstructure.

FIG. 4 is a characteristic view showing the result of an evaluation ofthe ferroelectric film that is a sample 1.

FIG. 5 is a drawing showing a representative XRD chart of c-axisoriented PZT.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the embodiment of the present invention will be explainedparticularly using the drawings. However, a person skilled in the artwould understand easily that the present invention is not limited to theexplanations below, but that the form and detail thereof are changeablevariously without deviating from the purport and the scope of thepresent invention. Accordingly, the present invention should not beconstrued as being limited to the described contents of the embodimentshown below.

FIG. 1 is a cross-sectional view showing schematically the ferroelectricfilm according to an aspect of the present invention.

The ferroelectric film according to the present embodiment is a(K_(1−x)Na_(x))NbO₃ film 12 having a perovskite structure, and Xpreferably satisfies a formula (1) below0.3≦X≦0.7.  (1)

Meanwhile, in the present embodiment, the (K_(1−x)Na_(x))NbO₃ film 12 isused as a ferroelectric film, but a BiFeO₃ film may be used as aferroelectric film. It is known that BiFeO₃ has good ferroelectrichysteresis, as was disclosed also in “Dielectric characteristics ofmultiferroic BiFeO₃ thin films,” The THIRD NANO TECH CENTER SEMINAR ofTHE INSTITUTE OF SCIENTIFIC AND INDUSTRIAL RESEARCH, OSAKA UNIVERSITY(Prediction, formation and evaluation by nano-technology, Nov. 1, 2006),and the film can be used as a ferroelectric film formed of a lead-freematerial because large ferroelectricity leads to large piezoelectricity.

On at least one of the upper side and lower side of the(K_(1−x)Na_(x))NbO₃ film 12, a crystalline oxide 11 or 13 is formed.

The crystalline oxides 11 and 13 are preferentially oriented to (001).The preferential orientation indicates the case where the strongest peakis twice or more as compared with the second strongest peak, amongrespective peaks in a Θ-2Θ analysis chart when X-ray structural analysisis carried out. A specific explanation will be given using, for example,FIG. 5. However, the XRD reflection intensity on the ordinate axis inFIG. 5 is of an analog intensity representation (cps), and a logarithmicrepresentation is not used. The next to the strongest peak (001) amongrespective peaks of the PZT is (110), and since there is a twice or moredifference in the strength, the (001) is referred to as the preferentialorientation.

By preferentially orienting the crystalline oxides 11 and 13 to (001),the (K_(1−x)Na_(x))NbO₃ film can be oriented to (001) and as a result,piezoelectric properties can be enhanced.

The crystalline oxides 11 and 13 are each preferably a bismuthlayered-structure ferroelectric substance having a pseudo-perovskitestructure or a tungsten-bronze type ferroelectric substance.

It is known that the bismuth layered-structure ferroelectric substancehas an anisotropic property in the crystal growth direction thereof dueto a unique crystal structure thereof (paper, Takeshi Kijima et al.:Jpn. J. Appl. Phys. 35 (1996) 1246). The bismuth layered-structureferroelectric substance has characteristics of being quicklypreferentially oriented to (001) to be crystallized (characteristics ofbeing oriented automatically to (001) that is the c-axis orientation, tobe crystallized, even on a Pt film oriented to, for example, (111)), andhas a crystal structure represented by a general formula(Bi₂O₂)²⁺(A_(m−1)B_(m)O_(3m+1))²⁻(m=1 to 5) or Bi₂A_(m−1)B_(m)O_(3m+3)(m=1 to 5), the structure in which a plurality of pseudo-perovskitestructures is interposed between (Bi₂O₂)²⁺ layers. Here, m shows thenumber of oxygen octahedrons in a pseudo-perovskite layer in a unitlattice. The ferroelectric substance is considered to exhibitferroelectricity through slight rotation or inclination of the oxygenoctahedron with the center of a B site ion, as in the case of atungsten-bronze type ferroelectric substance, and has largepiezoelectric anisotropy as in the case of the tungsten-bronzestructure.

As shown in FIG. 2, examples of the bismuth layered-structureferroelectric substances include Bi₄Ti₃O₁₂ being BIT or (Bi_(4−x)La_(x))Ti₃O₁₂ being BLT, and is, specifically, (Bi_(3.25)La_(0.75))Ti₃O₁₂.Meanwhile, x satisfies a formula below0≦x≦1.

The crystal structure of the tungsten-bronze type ferroelectricsubstance is a crystal structure found through studies on sodiumtungstate (Na_(x)WO₃), x=0.0 to 1.0. The sodium tungstate takes on goldcolor in the composition of x=1.0, and is generally referred to as thetungsten-bronze type crystal structure. A tungsten-bronze compound has abasic structure of A_(x)BO₃ (x<1), in which a part of A site ions in theperovskite structure ABO₃ is lost, and is represented generally by acomposition formula of a form in which the whole has been subjected toan integral multiple. In FIG. 3, a typical tungsten-bronze type crystalstructure is shown. A ferroelectric substance of the present structureexpresses ferroelectric properties through slight rotation orinclination of the oxygen octahedron constituted by six oxygen ions withthe center of a B site ion, along with the change in an electric field.As a typical lead-free piezoelectric ceramics material of thisstructure, there exists Ba₂NaNb₅O₁₅.

Next, the method for manufacturing a ferroelectric film according to thepresent embodiment will be explained in detail, while referring toFIG. 1. The ferroelectric film is formed of a perovskite structureferroelectric substance represented by (K_(1−x)Na_(x))NbO₃, X satisfyingthe above formula (1).

(Substrate)

On a substrate such as, for example, a 6-inch Si wafer, a foundationfilm oriented in a prescribed crystalline plane is formed. As thefoundation film, for example, a (111)-oriented Pt film or an Ir film isused.

Next, on the foundation film, a crystalline oxide 11 having previouslybeen preferentially oriented to (001) plane is formed. The crystallineoxide 11 may be a crystalline oxide in an island shape or in a filmshape.

Alternately, the crystalline oxide 11 may be one in which, by coating ofa known sol-gel solution for forming a crystalline oxide preferentiallyoriented to (001) by a spin-coating method, a coated film is formed onthe foundation film and by calcination of the coated film, a materialfilm for forming a crystalline oxide, formed of the coated film isdeposited on the foundation film. Meanwhile, by repeating a plurality oftimes the formation and calcination of the coated film, a material filmfor forming a crystalline oxide formed of a plurality of coated filmsmay be deposited.

Subsequently, a sol-gel solution for forming the (K_(1−x)Na_(x))NbO₃film 12 is prepared. The sol-gel solution contains a raw materialsolution including a hetero polyacid including K, Na and Nb, polarsolvents and unsaturated fatty acids. The total concentration of K, Naand Nb contained in the sol-gel solution is favorably 10 to 50 mol %.

The sol-gel solution has, as a constituent element, a hetero polyacidion having a Keggin-type structure in which the molecular structure iscaused to be noncentrosymmetric to thereby express nonlinearity, andcontains, as a part of a precursor structure of ferroelectric ceramics,a hetero polyacid ion in which at least one polyatom is lost or a heteropolyacid ion in which a part of the polyatom of the hetero polyacid ionhas been substituted by another atom.

The above-described hetero polyacid ion is one having a Keggin-typestructure as represented by a following general formula:[XM_(y)M′_(12−y)O₄₀]^(n−) (in the formula, X is a hetero atom, M is apolyatom, M′ is a polyatom different from M, n is a valence number, andy=1 to 11), and the hetero polyacid ion is contained as a part of aprecursor structure of ferroelectric ceramics.

Furthermore, the above-described hetero polyacid ion may be one having aKeggin-type structure represented by a general formula: [XM₁₁O₃₉]^(n−)(in the formula, X is a hetero atom, M is a polyatom, and n is a valencenumber), and the hetero polyacid ion is contained as a part of aprecursor structure of ferroelectric ceramics.

Moreover, the above-described hetero polyacid ion is one having aKeggin-type structure represented by a general formula:[XM_(z)M′_(11−z)O₃₉]^(n−) (in the formula, X is a hetero atom, M is apolyatom, M′ is a polyatom different from M, n is a valence number, andz=1 to 10), and the hetero polyacid ion is contained as a part of aprecursor structure of ferroelectric ceramics.

In the above-described hetero polyacid ion, it is also possible for thehetero atom to be selected from a group consisting of B, Si, P, S, Ge,As, Mn, Fe and Co and for the polyatom to be selected from the groupconsisting of Mo, V, W, Ti, Al, Nb and Ta, and the hetero polyacid ionmay be contained as a part of a precursor structure of ferroelectricceramics.

The polar solvents are any of methyl ethyl ketone, 1,4-dioxane,1,2-dimethoxyethane acetamide, N-methyl-2-pyrrolidone, acetonitrile,dichloromethane, nitromethane, trichloromethane, dimethylformamide,monomethylformamide or a plurality of combinations thereof.

The unsaturated fatty acid is any of mono-unsaturated fatty acids,di-unsaturated fatty acids, tri-unsaturated fatty acids,tetra-unsaturated fatty acids, penta-unsaturated fatty acids andhexa-unsaturated fatty acids or a plurality of combinations thereof.

Examples of the mono-unsaturated fatty acids include crotonic acid,myristoleic acid, palmitoleic acid, oleic acid, elaidic acid, vaccenicacid, gadoleic acid, eicosenoic acid, erucic acid and nervonic acid, andany of these or a plurality of combinations thereof may be used.

Examples of the di-unsaturated fatty acids include linoleic acid,eicosadienoic acid and docosadienoic acid, and any of these or aplurality of combinations thereof may be used.

Examples of the tri-unsaturated fatty acids include linolenic acid,pinolenic acid, eleostearic acid, Mead acid, dihomo-γ-linolenic acid andeicosatrienoic acid, and any of these or a plurality of combinationsthereof may be used.

Examples of the tetra-unsaturated fatty acids include stearidonic acid,arachidonic acid, eicosatetraenoic acid and adrenic acid, and any ofthese or a plurality of combinations thereof may be used.

Examples of the penta-unsaturated fatty acids include bosseopentaenoicacid, eicosapentaenoic acid, osbond acid, clupanodonic acid andtetracosapentaenoic acid, and any of these or a plurality ofcombinations thereof may be used.

Examples of the hexa-unsaturated fatty acids include docosahexaenoicacid and nisinic acid, and any of these or a plurality of combinationsthereof may be used.

Next, on the crystalline oxide 11 having previously been preferentiallyoriented to (001) or on the material film for forming a crystallineoxide, the sol-gel solution is coated. The result of measuring a contactangle of the sol-gel solution with the substrate gave 20° or less.Meanwhile, it suffices that the contact angle with a substrate is 1 to40° (preferably 1 to 20°).

The coating of the sol-gel solution is carried out by a spin-coatingmethod. Consequently, on the crystalline oxide 11 or the material filmfor forming a crystalline oxide, a coated film is formed, and bycalcination of the coated film at a temperature of 25 to 450° C.(preferably at a temperature of 450° C.), a (K_(1−x)Na_(x))NbO₃ materialfilm formed of the coated film is deposited on the crystalline oxide 11or the material film for forming a crystalline oxide. Meanwhile, byrepeating the formation and calcination of the coated film a pluralityof times, the (K_(1−x)Na_(x))NbO₃ material film formed of a plurality ofthe coated films may be deposited on the crystalline oxide 11 or thematerial film for forming a crystalline oxide.

Subsequently, on the (K_(1−x)Na_(x))NbO₃ material film, there isdeposited the crystalline oxide 13 having previously been oriented to(001) or the material film for forming a crystalline oxide. As thecrystalline oxide 13 or the material film for forming a crystallineoxide, there can be used the same one as the above-described crystallineoxide 11 or material film for forming a crystalline oxide.

(Crystallization Method)

By heat-treating the material film for forming a crystalline oxide, the(K_(1−x)Na_(x))NbO₃ material film and the material film for forming acrystalline oxide in an oxygen atmosphere at a temperature of 450 to900° C. (preferably 900° C.), these can be crystallized. As to thecondition at this time, the heat treatment may be carried out in apressure range of 0.0993 to 1.986 MPa. Furthermore, as to conditions ofthe heat treatment at this time, the calcination may be carried out for1 to 5 minutes in a pressurized oxygen atmosphere of 2 to 20 atm, at atemperature rising rate of 50 to 150° C./sec. Moreover, the thickness ofthe (K_(1−x)Na_(x))NbO₃ material film when crystallizing together the(K_(1−x)Na_(x))NbO₃ material film is preferably 300 nm or more.

The crystalline oxides 11 and 13 obtained by crystallizing the materialfilm for forming a crystalline oxide as described above arepreferentially oriented to (001), and, in the crystallized(K_(1−x)Na_(x))NbO₃ film 12, X preferably satisfies a formula (1) below0.3≦X≦0.7.  (1)

The (K_(1−x)Na_(x))NbO₃ film 12 scarcely contains an air bubble even ifit is a thick film with a thickness of 500 nm or more. In other words,by depositing a film according to this procedure, an excellent thickfilm can be formed. The reason is that the film is formed of a structurein which organic components disappear almost in the thickness direction,the film scarcely shrinks in the substrate surface, and the shrinkage isto the extent of being compensated by expansion due to oxidation.Accordingly, the substrate scarcely exhibits warpage.

Meanwhile, by repeating the formation and crystallization of the(K_(1−x)Na_(x))NbO₃ material film, it is also possible to form the(K_(1−x)Na_(x))NbO₃ film 12 with a thickness of 2 μm or more.

Furthermore, the total thickness of the crystalline oxides 11 and 13crystallized as described above is 1 to 30 nm, preferably 15 to 25 nm,and more preferably 20 nm.

Meanwhile, in FIG. 1, the crystalline oxides 11 and 13 are formed onboth the upper side and lower side of the ferroelectric film 12,respectively, but the crystalline oxide may be formed on at least one ofthe upper side and lower side of the ferroelectric film 12. When thecrystalline oxide is formed only on one of the upper side and lower sideof the ferroelectric film, the thickness of the crystalline oxide on oneside is 1 to 30 nm, preferably 15 to 25 nm, and more preferably 20 nm.

The crystal in the crystalline oxides 11 and 13 serves as a nucleus whencrystallizing the (K_(1−x)Na_(x))NbO₃ material film, and thus it becomespossible to proceed promptly with the crystallization of the(K_(1−x)Na_(x))NbO₃ material film that is difficult to be crystallizedinto the perovskite structure. Since the crystalline oxides 11 and 13act as the nucleus in the crystallization in this way, it suffices thatthe crystalline oxide has been formed on at least one of the(K_(1−x)Na_(x))NbO₃ material film.

When forming the crystalline oxide 11 only on the lower side of the(K_(1−x)Na_(x))NbO₃ material film, favorably a blocking film is formedon the upper side of the (K_(1−x)Na_(x))NbO₃ material film. Variousblocking films can be used as long as they function in order to suppressthe separation of K and Na from the (K_(1−x)Na_(x))NbO₃ material filmwhen the (K_(1−x)Na_(x))NbO₃ material film is heat-treated in an oxygenatmosphere and is crystallized.

Furthermore, the crystalline oxides 11 and 13 have preferably higherdielectric constant than the (K_(1−x)Na_(x))NbO₃ film 12 being aferroelectric film. The “higher dielectric constant” referred to heremeans that the whole dielectric constant of the crystalline oxides 11and 13 is higher than the whole dielectric constant of the ferroelectricfilm 12, that is, it means so-called real dielectric constant.Consequently, when a voltage is applied serially to the crystallineoxides 11 and 13, and the (K_(1−x)Na_(x))NbO₃ film 12, an electric fieldis added to the (K_(1−x)Na_(x))NbO₃ film 12 having lower dielectricconstant.

According to the present embodiment, the (K_(1−x)Na_(x))NbO₃ film 12that is a ferroelectric film formed of a lead-free material, can beproduced.

In addition, in the present embodiment, by using the crystalline oxides11 and 13 preferentially oriented to (001), a (K_(1−x)Na_(x))NbO₃ filmcan be crystallized, oriented to (001), and as a result, piezoelectricproperties of the (K_(1−x)Na_(x)) NbO₃ film can be enhanced.

Furthermore, according to the present embodiment, since the heattreatment for the crystallization is carried out in a state where the(K_(1−x)Na_(x))NbO₃ material film is sandwiched between the crystallineoxide 11 and the crystalline oxide 13, it is possible to suppress theescape of K and Na in the (K_(1−x)Na_(x))NbO₃ material film and it ispossible to improve film properties of the crystallized(K_(1−x)Na_(x))NbO₃ film 12.

Moreover, by adopting a pressurized oxygen atmosphere when carrying outthe heat treatment for crystallizing the (K_(1−x)Na_(x))NbO₃ materialfilm, it is possible to suppress the escape of K and Na in the(K_(1−x)Na_(x))NbO₃ material film and it is possible to improve filmproperties of the crystallized (K_(1−x)Na_(x)) NbO₃ film 12.

In addition, the crystalline oxides 11 and 13 may be removed aftercrystallizing the (K_(1−x)Na_(x))NbO₃ material film. As the removalmethod for example, an etching method is used.

Furthermore, in the present embodiment, as shown in FIG. 1, there isformed a ferroelectric film in which the crystalline oxide 11, the(K_(1−x)Na_(x))NbO₃ film 12 and the crystalline oxide 13 are stacked inthis order, but there may also be stacked a plurality of ferroelectricfilms that includes the (K_(1−x)Na_(x))NbO₃ film 12, and the crystallineoxide 11 formed on at least one of the upper side and lower sidethereof.

EXAMPLE

A Ti film of 10 to 30 nm is formed on a 6-inch Si wafer via a siliconoxide film by a sputtering method. Particularly, the Ti film was formedby an RF sputtering method. The Ti film serves as an adhesive layer ofplatinum and silicon oxide. The Ti film was formed under conditions suchas argon gas pressure of 0.2 Pa, power output of 0.12 kW and depositiontime of 20 minutes. The film formation was carried out at substratetemperature of 200° C.

Next, the Ti film is subjected to a heat treatment at a temperature of650° C. for 5 minutes, by RTA (Rapid Thermal Anneal). The treatment wascarried out in an oxygen atmosphere at 9.9 atm and at 100° C./sec.

Then, on the Ti film, a first Pt film of 100 nm is deposited by asputtering method at a temperature of 550 to 650° C. The deposition wascarried out at argon gas pressure of 0.4 Pa, at an output power of DCpower of 100 W and for a deposition time of 25 minutes.

Subsequently, on the first Pt film, a second Pt film of 100 nm isdeposited by an evaporation method at ordinary temperature. Thedeposition was carried out at 3.3×10⁻³ Torr, at an output power of 10kV, and for a deposition time of 4 minutes.

After that, the Si wafer is subjected to a heat treatment by RTA, at atemperature of 650 to 750° C. for 1 to 5 minutes. As described above, a6-inch Si wafer, on the surface of which a (111)-oriented Pt film isformed, is prepared.

Next, a material film of Bi₄Ti₃O₁₂ (Bi_(3.25)La_(0.75))Ti₃O₁₂ forforming a crystalline oxide preferentially oriented to (001) on the Siwafer is deposited. The deposition conditions at this time are asfollows.

As a sol-gel solution for forming a material film of Bi₄Ti₃O₁₂ or(Bi_(3.25)La_(0.75)) Ti₃O₁₂, there was used a sol-gel solution with aconcentration of 8% by weight, in which metal elements were mixed atBi:La:Ti=3.65:0.75:3 with 10% excessive Bi using n-butanol as a solvent,manufactured by TOSHIMA MFG Co., Ltd. Using the present solution,spin-coating formation of the material film of Bi₄Ti₃O₁₂ or(Bi_(3.25)La_(0.75)) Ti₃O₁₂ was carried out. As a spin coater, MS-A200manufactured by MIKASA CO., LTD was used. First, after the spin coaterwas rotated at 300 rpm for 5 seconds and at 2000 rpm for 30 to 60seconds, the rotation rate was gradually raised up to 3000 rpm and waskept for 10 seconds. Subsequently, the spin-coated wafer was left on ahot plate (ceramic hot plate AHS-300, manufactured by AS ONECorporation) at 200° C. for 0.5 minutes in the air, then the wafer wasleft on a hot plate (similarly AHS-300) at 450° C. for 1 minutesimilarly in the air, and after that, the wafer was cooled to roomtemperature. As described above, the material film of Bi₄Ti₃O₁₂ or(Bi_(3.25)La_(0.75)) Ti₃O₁₂ with a thickness of 20 nm was formed on theSi wafer.

Next, there is prepared a sol-gel solution, the contact angle of whichwith above-described material film of Bi₄Ti₃O₁₂ or(Bi_(3.25)La_(0.75))Ti₃O₁₂ is 40° or less, preferably 20° or less.Particularly, the sol-gel solution contains a raw material solutionincluding a hetero polyacid including K, Na and Nb, polar solvents andunsaturated fatty acids.

A raw material solution for forming the (K_(1−x)Na_(x)) NbO₃ film isformed of being mixed with a hetero polyacid that is a polyacid of a(X_(l)M_(m)O_(n))^(x−) type with a hetero atom inserted in a metaloxygen acid skeleton. The raw material solution is a sol-gel solutionfor forming an oxide film, in which the polyatom is formed of M=Mo, V,W, Ti, Al, Nb and Ta, and a hetero atom means elements other than H andC, and which is preferably formed of M=B, Si, P, S, Ge, As, Fe, Co andBi.

Polar solvents are any of methyl ethyl ketone, 1,4-dioxane,1,2-dimethoxyethane acetamide, N-methyl-2-pyrrolidone, acetonitrile,dichloromethane, nitromethane, trichloromethane, dimethylformamide andmonomethylformamide, or a plurality of combinations thereof.

Regarding unsaturated fatty acids, examples of mono-unsaturated fattyacids include crotonic acid, myristoleic acid, palmitoleic acid, oleicacid, elaidic acid, vaccenic acid, gadoleic acid, eicosenoic acid,erucic acid and nervonic acid, examples of di-unsaturated fatty acidsinclude linoleic acid, eicosadienoic acid and docosadienoic acid,examples of tri-unsaturated fatty acids include linolenic acid,pinolenic acid, eleostearic acid, Mead acid, dihomo-γ-linolenic acid andeicosatrienoic acid, examples of tetra-unsaturated fatty acids includestearidonic acid, arachidonic acid, eicosatetraenoic acid and adrenicacid, examples of penta-unsaturated fatty acids include bosseopentaenoicacid, eicosapentaenoic acid, osbond acid, clupanodonic acid andtetracosapentaenoic acid, and examples of hexa-unsaturated fatty acidsinclude docosahexaenoic acid and nisinic acid.

Next, by coating the sol-gel solution on the material film of Bi₄Ti₃O₁₂or (Bi_(3.25)La_(0.75)) Ti₃O₁₂ by a spin-coating method, a coated filmof a first layer is formed on the material film of the Bi₄Ti₃O₁₂ or(Bi_(3.25)La_(0.75)) Ti₃O₁₂. Particularly, the sol-gel solution of 500μL was coated, raised from 0 to 500 rpm in 3 seconds, held at 500 rpmfor 3 seconds, then rotated at 2000 rpm for 60 seconds, and after that,stopped.

Subsequently, the coated film of the first layer is heated with a hotplate at a temperature of 200° C. for 1 minute, and then, is calcined ata temperature of 450° C. for 1 minute. Consequently, on the materialfilm of Bi₄Ti₃O₁₂ or (Bi_(3.25)La_(0.75))Ti₃O₁₂, a ferroelectricmaterial amorphous film of a first layer with a thickness of 125 nm isformed.

After that, in the same way as that for the coated film of the firstlayer, on the ferroelectric material film of the first layer, a coatedfilm of a second layer is formed. Subsequently, in the same way as thatfor the coated film of the first layer, the coated film of the secondlayer is heated and calcined. Consequently, on the ferroelectricmaterial film of the first layer, the ferroelectric material film of thesecond layer with a thickness of 125 nm is formed.

Next, in the same way as that for the coated film of the second layer,on the ferroelectric material film of the second layer, a coated film ofa third layer is formed. Then, in the same manner as that for the coatedfilm of the first layer, the coated film of the third layer is heatedand calcined. Consequently, on the ferroelectric material film of thesecond layer, the ferroelectric material film of the third layer withthickness of 125 nm is formed. The repetition of this forms aferroelectric material film of twelve layers. As described above, theferroelectric material film formed of twelve layers with a thickness of1.5 μm can be deposited.

Subsequently, on the ferroelectric material film, the material film ofBi₄Ti₃O₁₂ or (Bi_(3.25)La_(0.75))Ti₃O₁₂ for forming a crystalline oxidepreferentially oriented to (001) is deposited. Conditions for depositingthe film at this time are the same as those for above-described materialfilm of Bi₄Ti₃O₁₂ or (Bi_(3.25)La_(0.75)) Ti₃O₁₂.

Next, by subjecting the ferroelectric material film, and the materialfilm of Bi₄Ti₃O₁₂ or (Bi_(3.25)La_(0.75))Ti₃O₁₂ to a heat treatment bypressurized RTA, the crystallization of these films forms a(K_(1−x)Na_(x))NbO₃ film being a ferroelectric film and a crystallizedfilm of Bi₄Ti₃O₁₂ or (Bi_(3.25)La_(0.75)) Ti₃O₁₂ having beenpreferentially oriented to (001). As to heat treatment conditions atthis time, the crystallization was carried out at a temperature risingrate of 100° C./sec up to 900° C. instantly in an oxygen atmospherepressurized at an oxygen partial pressure of 9.9 atm, and by holding thetemperature for one minute.

Meanwhile, in the present Example, the ferroelectric film of 1.5 μm isformed, but a ferroelectric film with thicker thickness or aferroelectric film with thinner thickness may by formed.

FIG. 4 is a characteristic view showing the result of evaluating theferroelectric film of a sample 1. Meanwhile, the abscissa axis in FIG. 4represents the applied voltage (Volts), and the ordinate axis in FIG. 4represents displacement (%).

FIG. 4 shows the result of performing the evaluation by driving theferroelectric film through the use of a bipolar pulse of ±10 V at afrequency of 700 Hz. As shown in FIG. 4, the ferroelectric film of thesample 1 was confirmed to have excellent piezoelectric properties.

REFERENCE SIGNS LIST

-   11: crystalline oxide-   12: (K_(1−x)Na_(x))NbO₃ film (ferroelectric film)-   13: crystalline oxide

The invention claimed is:
 1. A ferroelectric film comprising: a(K_(1−x)Na_(x))NbO₃ film or a BiFeO₃ film having a perovskite structure;and a crystalline oxide preferentially oriented to (001), formed on atleast one of an upper side and lower side of said (K_(1−x)Na_(x))NbO₃film or BiFeO₃ film, wherein said crystalline oxide is a bismuthlayered-structure ferroelectric substance having a pseudo-perovskitestructure or a tungsten-bronze type ferroelectric substance, whereinsaid bismuth layered-structure ferroelectric substance is (Bi₂O₂)²⁺(A_(m−1)B_(m)O₃₊₁)²⁻(m=1 to 5) or Bi₂A_(m−1)B_(m)O_(3m+3)(m=1 to 5),and wherein X satisfies the formula below0.3≦X≦0.7.
 2. The ferroelectric film according to claim 1, wherein saidcrystalline oxide is formed in an island shape or in a film shape. 3.The ferroelectric film according to claim 1, wherein a thickness of saidcrystalline oxide is 2 to 30 nm.
 4. The ferroelectric film according toclaim 1, wherein said (K_(1−x)Na_(x))NbO₃ film or BiFeO₃ film is formedby a sol-gel method.
 5. The ferroelectric film according to claim 1,wherein a ferroelectric film including said (K_(1−x)Na_(x))NbO₃ film orBiFeO₃ film, and said crystalline oxide formed on at least one of theupper side and lower side thereof is stacked.
 6. A ferroelectric filmcomprising: a (K_(1−x)Na_(x))NbO₃ film or a BiFeO₃ film having aperovskite structure; and a crystalline oxide preferentially oriented to(001), formed on at least one of an upper side and lower side of said(K_(1−x)Na_(x))NbO₃ film or BiFeO₃ film, wherein said crystalline oxideis a tungsten-bronze type ferroelectric substance, and wherein Xsatisfies the formula below0.3<X<0.7.
 7. The ferroelectric film according to claim 6, wherein saidcrystalline oxide is formed in an island shape or in a film shape. 8.The ferroelectric film according to claim 6, wherein a thickness of saidcrystalline oxide is 2 to 30 nm.
 9. The ferroelectric film according toclaim 6, wherein said (K_(1−x)Na_(x))NbO₃ film or BiFeO₃ film is formedby a sol-gel method.
 10. The ferroelectric film according to claim 6,wherein a ferroelectric film including said (K_(1−x)Na_(x))NbO₃ film orBiFeO₃ film, and said crystalline oxide formed on at least one of theupper side and lower side thereof is stacked.
 11. A ferroelectric filmcomprising: a (K_(1−x)Na_(x))NbO₃ film or a BiFeO₃ film having aperovskite structure; and a crystalline oxide preferentially oriented to(001), formed on at least one of an upper side and lower side of said(K_(1−x)Na_(x))NbO₃ film or a BiFeO₃ film, wherein X satisfies theformula below0.3<X<0.7, wherein said crystalline oxide is a bismuth layered-structureferroelectric substance having a pseudo-perovskite structure or atungsten-bronze type ferroelectric substance, and wherein said bismuthlayered-structure ferroelectric substance is Bi₄Ti₃O₁₂ or(Bi_(4−x)La_(x))Ti₃O₁₂, and x satisfies the formula below0<x<1.
 12. The ferroelectric film according to claim 11, wherein saidcrystalline oxide is formed in an island shape or in a film shape. 13.The ferroelectric film according to claim 11, wherein a thickness ofsaid crystalline oxide is 2 to 30 nm.
 14. The ferroelectric filmaccording to claim 11, wherein said (K_(1−x)Na_(x))NbO₃ film or BiFeO₃film is formed by a sol-gel method.
 15. The ferroelectric film accordingto claim 11, wherein a ferroelectric film including said(K_(1−x)Na_(x))NbO₃ film or BiFeO₃ film, and said crystalline oxideformed on at least one of the upper side and lower side thereof isstacked.